BuildSystem: Enable PortAudio in NetBSD.

In NetBSD, PortAudio 1 is still the default version. PortAudio 2 can be
installed side by side but gets placed in a 'portaudio2' subdirectory. To
find PortAudio 2 even in a subdirectory, the tool pkg-config is queried via
AST_PKG_CONFIG_CHECK. For those platforms, which do not list PowerAudio 2
via pkg-config, the previous check remains and is executed thereafter.

ASTERISK-27721

Change-Id: I4175500126909ad1b181fff8e11bb4a3a6ae4fa9